The invention relates to a sensor with a temperature-dependent measuring element for temperature measurement, which is arranged in the interior of one end of a quartz glass tube, which faces the measurement, wherein a connection line of the measuring element is guided to the outside through the other end of the quartz glass tube.
From British patent specification GB 22 23 100 A, a high-temperature platinum resistance thermometer is known, in which a platinum coil is arranged as a measuring element in the closed end of a quartz tube. It has proven to be problematic that there is the possibility of an eventual contamination of the platinum sensor by the migration of metal vapor through the quartz wall of the sheathing tube. Furthermore, a quartz sheathing of sensors is considered to be susceptible to mechanical damage during robust usage in ovens, e.g., in household baking ovens.
Furthermore, from U.S. Pat. No. 4,538,927 a temperature sensor is known, which has as the outer sheathing a sleeve made of stainless steel with a thin wall thickness, which is closed off with a cap on the end facing the measurement. In the interior of the sheathing a mounting body made of plastic foam is provided, which not only functions for holding a measuring probe, but also functions for a convection interruption between the measuring probe and the end of the temperature sensor facing the cap. The relatively low heat resistance of the plastic foam in a range up to 140xc2x0 C. has proven to be problematic, so that use in ovens and/or household baking ovens is not possible.
Furthermore, from German published patent application DE-OS 1 473 279 a protective tube arrangement is known for rapid immersion measurement of a molten metal, in particular for measuring the casting temperature of molten iron, steel or the like, in which a protective tube element made of a heat-resistant material, in particular quartz, is provided with a thin wall thickness and a small outer diameter and, furthermore, with a closed off end, which has, in the interior of the closed end, a measuring element for the temperature measurement of the melt. In addition, a sheathing tube element made of a heat-insulating, non-combustible material, preferably quartz, is provided, which encloses the largest part of the length of the protective tube element with the exception of its closed end. Furthermore, a carrier tube element is provided as a holder in a manner known per se, which is shoved onto the sheathing tube element and, to be precise, on the end piece turned away from the closed end of the protective tube. A ring-shaped intermediate space located between the protective tube element and the sheathing tube element is preferably filled with a powder made of a heat-insulating, heat-resistant material. The relatively complex construction has proven to be problematic in a protective tube arrangement of this type.
From U.S. Pat. No. 4,717,787 a thermoelement is known for rapid temperature recording and defense against interference signals during the temperature measurement in vacuum ovens, wherein the protective tube is made of quartz glass and has a grounded metal wire wound around it. The winding with the grounded metal wire protects the thermoelement from electric interference effects in the oven space, without the radiation permeability of the protective tube being impaired. As wire materials, chromium-nickel, molybdenum and platinum have proven useful, depending on the temperatures of use. This also involves a relatively complex arrangement.
An object of the invention is to create a cost-effective sensor for temperature measurement, in particular in baking ovens for baking and cooking operations, whose insulation is largely voltage-stable. The temperature range provided for measurement therein should lie in a range of xe2x88x9240xc2x0 C. to 550xc2x0 C.
The object is achieved according to the invention in that the quartz glass tube is surrounded by a protective tube made of metal, which is closed off at its end facing the measurement with a rotationally symmetric formed body, wherein a spacer made of electrically insulating material is provided between the formed body and the measuring element. Advantageous embodiments of the invention are set forth below and in the dependent claims.
In a preferred embodiment of the invention, the rotationally symmetric formed body is pressed into (friction-set in) the open end of the protective tube and is thereby held form-fittingly. In an especially advantageous embodiment of the invention, the rotationally symmetric formed body is constructed as a sphere. However, it is also possible, for example, to use an ellipsoid or a cone-shaped body as a rotationally symmetric formed body. In an advantageous manner, the protective tube as well as the sphere are both made of stainless steel. As a stainless steel sphere, preferably a sphere according to DIN 5401, Workpiece 1.4034/1.3541 is used.
As the measuring element, preferably a platinum thin-layer resistor is used, whose connection lines are welded on and are electrically insulated from each other inside the quartz glass tube by a longitudinally extending intermediate connecting piece. Advantageously, the intermediate connecting piece also functions for the form-fit mounting of the measuring element. Both the spacer between the formed body or the sphere and the measuring element, as well as the intermediate connecting piece, are preferably made of mica.
It has proven to be advantageous that manufacturing and marking of a complete protective tube as well as the lock-beading of the connection cable in the protective tube are possible in a single work operation. Further, it has proven to be advantageous that the connection cable is continuous from the connection plug up to the measuring element without additional connection points.
Furthermore, a quartz glass tube may be manufactured in a relatively cost-effective manner. In addition, the use of quartz glass ensures the electrical insulation of the temperature sensor from the oven power supply, especially at higher temperatures. The voltage stability or the insulation between the electrical conductors and the protective tube is so high that, for example, when a voltage of 3000V is applied, no voltage arc-over occurs.
When using mica strips, it has proven to be advantageous that they are not very sensitive to breaking, and the mounting can be done radially, so that no insulation parts are threaded, which would then stand in the way when forming the connection point of the measuring element/connection line. The mica strip has the further advantage that it ensures an insulation separation distance from the formed body or from the metal sphere by a saving in the length of the measuring element and by the overhang forwardly beyond the measuring element.
A further advantage can be seen in the fact that a formed body constructed as a stainless steel sphere can be processed in an especially easy manner, since the sphere need not be exactly oriented in its position within the opening of the protective tube prior to its assembly.